PAD4 is a member of the peptidylarginine deiminase (PAD) family of enzymes capable of catalysing the citrullination of arginine into citrulline within peptide sequences. PAD4 is responsible for the deimination or citrullination of a variety of proteins in vitro and in vivo, with consequences of diverse functional responses in a variety of diseases (Jones J. E. et al, Curr. Opin. Drug Discov. Devel., 12(5), (2009), 616-627). Examples of exemplar diseases include rheumatoid arthritis, diseases with neutrophilic contributions to pathogenesis (for example vasculitis, systemic lupus erythematosus, ulcerative colitis) in addition to oncology indications. PAD4 inhibitors may also have wider applicability as tools and therapeutics for human disease through epigenetic mechanisms.
Inhibitors of PAD4 may have utility against Rheumatoid Arthritis (RA). RA is an auto-immune disease affecting approximately 1% of the population (Wegner N. et al, Immunol. Rev., 233(1) (2010), 34-54). It is characterised by inflammation of articular joints leading to debilitating destruction of bone and cartilage. A weak genetic association between PAD4 polymorphisms and susceptibility to RA has been suggested, albeit inconsistently, in a number of population studies (Kochi Y. et al, Ann. Rheum. Dis., 70, (2011), 512-515). PAD4 (along with family member PAD2) has been detected in synovial tissue where it is responsible for the deimination of a variety of joint proteins. This process is presumed to lead to a break of tolerance to, and initiation of immune responses to, citrullinated substrates such as fibrinogen, vimentin and collagen in RA joints. These anti-citrullinated protein antibodies (ACPA) contribute to disease pathogenesis and may also be used as a diagnostic test for RA (e.g. the commercially available CCP2 or cyclic citrullinated protein 2 test). In addition, increased citrullination may also offer additional direct contributions to disease pathogenesis through its ability to affect directly the function of several joint and inflammatory mediators (e.g. fibrinogen, anti-thrombin, multiple chemokines). In a smaller subset of RA patients, anti-PAD4 antibodies can be measured and may correlate with a more erosive form of the disease.
PAD4 inhibitors may also be useful for the reduction of pathological neutrophil activity in a variety of diseases. Studies suggest that the process of Neutrophil Extracellular Trap (NET) formation, an innate defence mechanism by which neutrophils are able to immobilise and kill pathogens, is associated with histone citrullination and is deficient in PAD4 knockout mice (Neeli I. et al, J. Immunol., 180, (2008), 1895-1902 and Li P. et al, J. Exp. Med., 207(9), (2010), 1853-1862). PAD4 inhibitors may therefore have applicability for diseases where NET formation in tissues contributes to local injury and disease pathology. Such diseases include, but are not limited to, small vessel vasculitis (Kessenbrock K. et al, Nat. Med., 15(6), (2009), 623-625), systemic lupus erythematosus (Hakkim A. et al, Proc. Natl. Acad. Sci. USA, 107(21), (2010), 9813-9818 and Villanueva E. et al, J. Immunol., 187(1), (2011), 538-52), ulcerative colitis (Savchenko A. et al, Pathol. Int., 61(5), (2011), 290-7), cystic fibrosis, asthma (Dworski R. et al, J. Allergy Clin. Immunol., 127(5), (2011), 1260-6), deep vein thrombosis (Fuchs T. et al, Proc. Natl. Acad. Sci. USA, 107(36), (2010), 15880-5), periodontitis (Vitkov L. et al, Ultrastructural Pathol., 34(1), (2010), 25-30), sepsis (Clark S. R. et al, Nat. Med., 13(4), (2007), 463-9), appendicitis (Brinkmann V. et al, Science, 303, (2004), 1532-5), and stroke. In addition, there is evidence that NETs may contribute to pathology in diseases affecting the skin, eg in cutaneous lupus erythematosis (Villanueva E. et al, J. Immunol., 187(1), (2011), 538-52) and psoriasis (Lin A. M. et al., J. Immunol., 187(1), (2011), 490-500), so a PAD4 inhibitor may show benefit to tackle NET skin diseases, when administered by a systemic or cutaneous route. PAD4 inhibitors may affect additional functions within neutrophils and have wider applicability to neutrophilic diseases.
Studies have demonstrated efficacy of tool PAD inhibitors (for example chloro-amidine) in a number of animal models of disease, including collagen-induced arthritis (Willis V. C. et al, J. Immunol., 186(7), (2011), 4396-4404), dextran sulfate sodium (DSS)-induced experimental colitis (Chumanevich A. A. et al, Am. J. Physiol. Gastrointest. Liver Physiol., 300(6), (2011), G929-G938), spinal cord repair (Lange S. et al, Dev. Biol., 355(2), (2011), 205-14), and experimental autoimmune encephalomyelitis (EAE). The DSS colitis report also demonstrates that chloro-amidine drives apoptosis of inflammatory cells both in vitro and in vivo, suggesting that PAD4 inhibitors may be effective more generally in widespread inflammatory diseases.
PAD4 inhibitors may also be useful in the treatment of cancers (Slack. J. L. et al, Cell. Mol. Life Sci., 68(4), (2011), 709-720). Over-expression of PAD4 has been demonstrated in numerous cancers (Chang X. et al, BMC Cancer, 9, (2009), 40). An anti-proliferative role has been suggested for PAD4 inhibitors from the observation that PAD4 citrullinates arginine residues in histones at the promoters of p53-target genes such as p21, which are involved in cell cycle arrest and induction of apoptosis (Li P. et al, Mol. Cell Biol., 28(15), (2008), 4745-4758).
The aforementioned role of PAD4 in deiminating arginine residues in histones may be indicative of a role for PAD4 in epigenetic regulation of gene expression. PAD4 is the primary PAD family member observed to be resident in the nucleus as well as the cytoplasm. Early evidence that PAD4 may act as a histone demethyliminase as well as a deiminase is inconsistent and unproven. However, it may reduce histone arginine methylation (and hence epigenetic regulation associated with this mark) indirectly via depletion of available arginine residues by conversion to citrulline. PAD4 inhibitors may therefore be useful as epigenetic tools or therapeutics for affecting expression of varied target genes in additional disease settings. Through such mechanisms, PAD4 inhibitors may also be effective in controlling citrullination levels in stem cells and may therefore therapeutically affect the pluripotency status and differentiation potential of diverse stem cells including, but not limited to, embryonic stem cells, neural stem cells, haematopoietic stem cells and cancer stem cells.